1. Field of the Invention
The present invention relates to digital image sensors, and, more specifically, to a chromatic adaptation method for maintaining color constancy under different illuminations in a digital image sensor.
2. Brief Description of the Related Art
Color constancy is one of the characteristics of the human vision system. The color appearance of the same object looks approximately identical under vastly different types of natural and artificial light sources, such as sun light and moon light, and incandescent, fluorescent, and candle light. The ability of the human visual system to determine surface colors under this wide range of illumination conditions is called constancy. In electronic imaging systems, this is commonly implemented with limited success as an automatic white balance. Extensive research has been conducted into ways to achieve human eye-like color constancy in electronic image sensors. Nevertheless, present day white balance systems not only lack a response sufficiently similar to that of the human eye, they also achieve only a narrow subset of the overall needs of a true color constancy system.
One of the ways that the human visual system achieves constancy is referred to as adaptation, which can be understood as a change in gain of the signal from the cone receptors of the eye. The cone receptors become less sensitive because of chemical bleaching in the presence of increased light. This results in a reduction in sensitivity for those cones experiencing greater light intensities. If the light is strongly colored, then the different cone types will become differentially adapted. In red light, for example, long wavelength cones will become less sensitive. The effect of adaptation is to make the eye have a sensitivity range appropriate to the environment.
This theory of constancy in the human vision system generally holds that differences in the type of illumination are accommodated by the chromatic adaptation of the human vision system. The sensitivities of long (L), middle (M) and short (S) wavelength cones adapt to stimuli in a largely independent way. This is the hypothesis proposed by von Kries, although exact details of the adaptation were not provided. (“Chromatic Adaptation,” J. von Kries, Festschrift der Albrecht-Ludwig-Universität, 1902).
Examples of some of the algorithms that have been explored for providing color constancy in electronic image sensors include: Gray World, Retinex, Gamut Mapping Methods, Color by Correlation, and Neural Net Methods. See “A Comparison of Computational Color Constancy Algorithms,” Parts One and Two, by K. Barnard et al., available at http://www.cs.berkeley.edu/˜kobus/research/publications/comparison{—1 or —2}/comparison{—1 or —2}.pdf.
Most electronic image sensors are designed with spectral responses that evenly divide the visible spectrum into color ranges, such as the three primary colors red, blue, and green, with little or no overlap between each range. The response represents the absolute photon acquisition experienced by each pixel of the digital image sensor, for example.
In contrast to the known electronic image sensors, the three types of color receptors in the human eye—long-, middle-, and short-wavelength cones (LMS)—have been found to exhibit significant overlap in spectral response. As a consequence of this overlapping spectral response, the hue-discrimination response of the human eye is highly non-linear, with peak sensitivity occurring near certain wavelengths. By comparison, an imaging array that utilizes an RGB filter, such as a Bayer filter, acts as a simplified band pass filter that does not correspond to the spectral response of the human eye.
Color standards are maintained by the Commission Internationale de L'Eclairage (CIE). The CIE has developed standard color systems based on the concept of a standard observer. The standard observer is based on a model of human rods and cones. The CIE system does not take adaptation into account, however. The CIE systems define color using tristimulus values X, Y, and Z. Y is the same as luminance (black and white).
It would be desirable to have an imaging system which more nearly replicates the color discrimination of the human eye to achieve more constancy in color reproduction under different lighting conditions.